In the past the demand for ultra high purity (UHP) oxygen of greater than 99.5% has been sporadic and required only limited quantities. Two principal methods produced ultra high purity oxygen sufficient to meet this demand.
The first method is the operation of a conventional air separation plant at greatly reduced UHP oxygen product recovery rates. The plant can be any one of several designs, such as the classical Linde dual-column configuration for either liquid oxygen (LOX) or gaseous oxygen (GOX). The plant is operated at an increased air feedrate such that the resulting reflux ratios in the low-pressure column yield the required purity utilizing the available tray configuration. One drawback of this method is the high specific power required. Another drawback is that crude argon cannot be economically produced.
The second method is the operation of a plant specifically designed to increase the usual commercial grades of liquid oxygen to the required purity. This plant would normally consist of distillation columns and a heat pump system to operate the columns, with necessary heat exchangers. An example of this method is more fully disclosed in U.S. Pat. No. 3,363,427.
In addition to the two principal methods detailed above, U.S. Pat. No. 3,969,481 describes the electrolysis of water with subsequent drying and purification to produce ultra high purity oxygen.
The rectification of a gas mixture containing at least three components is shown in U.S. Pat. No. 2,817,216 ('216). The process of the '216 patent increases the purity of the lower boiling component, specifically nitrogen, by increasing the yield of the intermediate boiling point component(s), specifically argon, utilizing various recycle streams. In one embodiment, a nitrogen recycle compressor is shown on the high-pressure column. Patentee notes generally that increasing the yield of the intermediate component will also increase the purity of the higher boiling point component.
With the advent of the space age and the related technology that has grown around it, there has been a marked increase in demand for ultra high purity oxygen. One important factor leading to the increased demand has been the use of ultra high purity oxygen in fuel cells.
All of the methods of producing ultra high purity oxygen described above require a high specific power. Power is the prime cost of producing ultra high purity oxygen. In order to reduce the costs of processes that utilize ultra high purity oxygen as a feed stream, the cost of producing ultra high purity oxygen must be reduced.